1 Answer
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Because there is no physical redundancy in the description. A gauge theory (and related gauge transformations) only occurs if there are different field configurations corresponding to a certain physical configuration. Such redundant configurations can be related by local gauge transformations (see for example the vector potential $A_\mu$ in electrodynamics). In your case (which the other question referred to), the Lagrangian is not invariant under your transformation, since the quantity
$$U(\phi)= \frac{1}{8} \phi^2 (\phi -2)^2$$
changes under $\phi\rightarrow\phi-1$ to